Primary and secondary manufacturing
Primary manufacturing turns raw materials into standard stock: steel mills (ore → sheet, bar, tube), sawmills (logs → boards), refineries and chemical plants (crude oil → plastic pellets), smelters (bauxite → aluminium). Secondary manufacturing turns stock into finished products: cars, furniture, phones, packaging.
Manufacturing systems: custom/job (one-off), batch (a set number), continuous/mass (non-stop line). Many factories now use automation, robots and just-in-time supply. Careers range from machine operator and welder to technician, technologist and engineer.
Material conversion processes
- Forming: shape changes, no material removed. Casting, forging, rolling, bending, extrusion, injection moulding.
- Separating: material removed. Sawing, drilling, turning (lathe), milling, grinding, shearing, laser and plasma cutting.
- Combining: parts joined or mixed. Welding (MIG, TIG, arc), soldering, brazing, fasteners (bolts, rivets, screws), adhesives, composites.
- Conditioning: inner properties changed. Hardening, tempering, annealing, case hardening.
- Finishing: surface protected or improved. Painting, powder coating, plating, polishing.
Tools and machines
Hand tools (files, hacksaws, taps and dies), power tools (drills, grinders), machine tools (lathe, milling machine, drill press) and CNC machines that follow a program. Choose a tool by material, accuracy needed, number of parts and safety. Always: guards on, safety glasses, hair tied back, no loose clothes, stop the machine before measuring.
Materials: properties and selection
Material families: ferrous metals (contain iron: mild steel, cast iron, stainless steel), non-ferrous metals (aluminium, copper, brass), polymers (thermoplastics that re-melt, thermosets that do not), woods, ceramics and composites (carbon fibre, fibreglass).
Key properties: strength (resists force), hardness (resists scratching), toughness (resists cracking), ductility (can be drawn into wire), malleability (can be hammered), density, corrosion resistance, thermal and electrical conductivity.
Select by: function (what forces and conditions), manufacturability (can our processes shape it?), cost and availability, and environment (recycling, energy to make). Prepare stock by marking out, cutting to size with an allowance, deburring and cleaning.
Drawings, process plans, metrology and control
Technical drawings
Orthographic projection shows front, top and side views. Dimensions are in millimetres. A tolerance such as 25.00 ± 0.05 mm gives the allowed range. Hidden edges use dashed lines. Drawings are now often made in CAD.
Process plan
A table of operations in order: operation, machine, tool, speed/feed, check. Example for a pin: cut stock → face end → turn diameter → chamfer → part off → deburr → measure.
Metrology
Steel rule (0.5 mm), vernier caliper (0.02 mm), micrometer (0.01 mm), dial indicator, go/no-go gauges. Vernier reading = main scale + (matching vernier line × least count). Compare the reading with the tolerance: inside = pass.
Maths and science on the shop floor
Spindle speed N = 1000·V/(π·D), with V the cutting speed in m/min and D in mm. Unit conversion (1 inch = 25.4 mm), percentages for waste, and material science of heat treatment.
Control systems
Open loop: input → process → output, no checking (a simple timer). Closed loop: a sensor measures the output and feeds it back, so the controller corrects errors (a CNC servo, a thermostat on a furnace). PLCs and CNC programs (G-code) run modern machines.
Try it
Take a cardboard box or a toy block. Sketch its front, top and side views on squared paper and add sizes in mm measured with a ruler. Then write a 5-line process plan to make it from a flat sheet. In the 3D, step 5, slide the part size until the lamp turns red.
Key formulas and definitions
- Primary: raw material → stock; Secondary: stock → product
- Forming, Separating, Combining, Conditioning (+ Finishing)
- Vernier reading = main scale + (n × least count)
- Least count of a common caliper = 0.02 mm; micrometer = 0.01 mm
- Accept if nominal − tol ≤ reading ≤ nominal + tol
- Spindle speed N = 1000 × V ÷ (π × D) rev/min
- Closed loop = open loop + sensor feedback
Worked examples
1. Classify: a sawmill making planks and a furniture factory making chairs.
Sawmill: primary (logs → standard boards). Furniture factory: secondary (boards → finished chairs).
2. A caliper's main scale reads 23 mm and the 7th vernier line matches. Least count 0.02 mm. Find the reading.
23 + 7 × 0.02 = 23.14 mm.
3. Drawing says 25.00 ± 0.05 mm. Readings: 24.96, 25.07, 25.04. Which pass?
Limits are 24.95 to 25.05 mm. 24.96 and 25.04 pass; 25.07 is rejected.
4. Find spindle speed to turn a 50 mm steel bar at a cutting speed of 30 m/min.
N = 1000 × 30 ÷ (π × 50) = 30000 ÷ 157.1 ≈ 191 rev/min.
5. Name the process: (a) pouring molten aluminium into a sand mould, (b) MIG welding a frame, (c) heating a chisel and quenching it.
(a) forming (casting), (b) combining, (c) conditioning (hardening).
6. Choose a material for an outdoor railing near the sea and give two reasons.
Stainless steel (or aluminium): high corrosion resistance in salty air, enough strength, and recyclable.
Common mistakes
- Calling cutting a 'forming' process. Forming keeps all the material; cutting (separating) removes it.
- Reading the vernier line number as the decimal directly. Multiply it by the least count: 7 × 0.02 = 0.14 mm.
- Using the cutting speed in m/min without the factor 1000 when D is in mm.
- Thinking closed-loop and open-loop differ by power. The difference is the feedback sensor.